Such temperature control devices are known in conjunction with radiant heaters installed in glass ceramic top cookers. The devices typically incorporate a rod-like temperature sensor that operates on the basis of a difference in thermal expansion coefficients between an expansion member of the sensor and a reference member of the sensor. The difference in thermal expansion gives rise to differential movement which, in turn, is employed to operate one or more switches which are used, for example, to limit the temperature of the glass ceramic cooking surface and/or to indicate that the surface of the glass ceramic may be too hot to be touched. The temperature sensor generally comprises a rod of high expansion material located coaxially within a tube of low expansion material such as quartz (more correctly known as fused silica). Alternatively, the temperature sensor may comprise a rod of low expansion material located coaxially within a tube of high expansion material.
Where at least two heating elements are incorporated in the radiant heater it is known, for example from United Kingdom Patent Specification No. 2 069 300, to isolate the temperature sensor from the heating effect of all except one of the heating elements. This can be achieved by enclosing part of the temperature sensor in a thermal insulation material or in a thermally conductive material which transmits the heat away to a heat sink, or by limiting the effective length of the temperature sensor to that part which extends over the relevant heating element. This latter means for isolating the temperature sensor can be put into effect by connecting part of the temperature sensor across the heating elements from which it is to be isolated in a manner which precludes those heating elements from influencing the response given by the sensor.
United Kingdom Patent Specification No. 2 080 660 also discloses a radiant heater having at least two heating elements wherein the temperature sensor is isolated from the heating effect of all but one of the heating elements. According to United Kingdom Patent Specification No. 2 080 660 this is achieved by extending the heating effect of the one heating element so as to influence substantially the entire effective length of the temperature sensor. United Kingdom Patent Specification No. 2 133 879 discloses means for isolating part of the temperature sensor from heat emitted by the heating elements of a radiant heater. According to United Kingdom Patent Specification No. 2 133 879 the temperature sensor comprises a rod of high thermal expansion material arranged coaxially within a tube which is assembled from at least two tubular sections. The tubular sections have different thermal expansions such that the overall thermal expansion of the whole tube is less than the thermal expansion of the rod. In practice, the rod is made of an iron-chromium alloy, one of the tubular sections is made of quartz glass or ceramic material and the other tubular section is made of the same material as the rod. Whilst such a temperature sensor can undoubtedly be isolated from all but the chosen heating element, the drawback is that the tubular section or sections where the temperature sensor is isolated are made of a relatively expensive material which requires to be machined to the required tubular shape. Additionally, because one or more of the tubular sections is or are made of metal and reduces the electrical clearance distance between the electrically live heating coil and the underside of the glass ceramic cooking surface, it is necessary electrically to insulate the metal section. This is achieved by surrounding the metal section with a tube of electrically insulating material, such as quartz glass or other ceramic material, which adds to the cost of the temperature control device. The result is that the temperature control device is not economic to manufacture.
European Patent Specification No. 0 141 923 also discloses means for isolating part of the temperature sensor. According to European Patent Specification No. 0 141 923 the temperature sensor comprises a tube of high thermal expansion material such as high-quality steel having arranged therein a rod made of at least two sections. One of the rod sections is made of a ceramic material, but the remaining section or sections are made of a material having a coefficient of thermal expansion at least as high, and preferably higher than that of the tube, thus providing a form of over-compensation in the response of the sensor to variations in temperature. In order to provide electrical isolation for the metal tube it is necessary to provide a further tube of quartz glass around the entire length of the metal tube. Such a temperature sensor has the disadvantage that the further tube acts as a heat sink and causes the temperature response of the sensor to lag behind the actual temperature. Moreover, the further tube constitutes an additional component which adds to the cost of the temperature control device and renders it uneconomic to manufacture.
Despite these drawbacks of known temperature control devices which are intended to provide temperature compensation for the sensor where it passes over heating elements from which it is required to be thermally isolated, such devices avoid the need for a block of thermal insulation material and represent an aesthetically appealing solution to the problem of achieving such isolation.